1. Field of the Invention
The invention relates to an image reading apparatus including a reading table having a transparent plate on which a document with an image thereon is placed, so that the document is irradiated with light transmitted through the transparent plate, to read the image thereby. The invention particularly relates to an image reading apparatus capable of detecting entrance of external light into a reading table.
2. Description of Related Art
As an image reading apparatus incorporated in a copy machine, a scanner, or a multifunctional apparatus having functions thereof, there are known a flatbed scanner, and an apparatus in which an image reading device, e.g., a CCD (Charge Coupled Device) or a CIS (Contact Image Sensor), is disposed at a predetermined relative position with respect to a mechanism called ADF (Auto Document Feeder) that operates to automatically feed document sheets, which is a document in the form of cut sheets, from a document supply tray to a catch tray along a feed path, in order to read an image on each document sheet while the document sheet is fed.
For instance, there is known an image reading apparatus that has a cover structure or a document holding member including an ADF, a reading table having at an upper surface thereof a platen glass, an image reading device disposed in the reading table, and a document supply tray on which document sheets each with an image thereon are stacked. On the platen glass, a fed-document reading area is defined, and the ADF feeds the document sheets one by one from the document supply tray to the fed-document reading area, so that the image reading device in the reading table operates to read the image on the document sheet as passing across the fed-document reading area on the platen glass. However, the image reading apparatus can also function as a flatbed scanner, that is, a stationary-document reading area is also defined on the platen glass, and the image reading apparatus can be used in such a manner that a document with an image thereon is placed on the platen glass and the document holding member is closed to hold down and fix in position the document, and then the image reading device is operated to read the image on the document.
The CCD or CIS used as an image reading device or image sensor has a light source and a light receiving element, and reads an image on a document as follows. The light source emits light toward the platen glass so that the emitted light is transmitted through the platen glass and reflected by a surface of the document. The reflected light is received by the light receiving element that converts the received light into electrical signals. Since undesirable variations occur with regard to operations of the light source and the light receiving element of such an image reading device, namely, an amount of light emitted from the light source and a spatial distribution of the light, and a photographic sensitivity of the light receiving element, a control operation called “shading correction” is implemented to correct image data obtained by reading the image on the document using the image reading device. More specifically, the shading correction is an operation such that the amount of the light as emitted from the light source is adjusted with respect to a white-colored reference member, and then the reference member is read to obtain white level data and black level data while the light source emits light of the amount adjusted as described above, so that the image data read thereafter is corrected based on the thus obtained white and black level data. For instance, the conventional image reading apparatus described above has the reference member on an under side of a partitioning member that is disposed to divide an upper surface of the platen glass into two areas, namely, the fed-document reading area and a stationary-document reading area, and the apparatus is set to adjust the amount of the light emitted from the light source with respect to the reference member, and obtain reference data, i.e., the white and black level data, prior to reading the image. It is noted that the amount of the light as emitted from the light source may be adjusted by adjusting an intensity of the light emitted therefrom.
Meanwhile, with improvement in the sensitivity of the image reading devices, it has recently become possible to use a light source of low illuminance. For instance, a CIS may employ as the light source a LED (Light Emitting Diode), or a LIDE (LED InDirect Exposure) using a LED and a linear light guide. Due to such lowering in the illuminance of the light sources, there arises a problem that reading of the reference member is significantly affected by external light. The term “external light” refers to indoor or outdoor light that enters the reading table of the image reading apparatus from the fed-document reading area or other places, when a document is placed within the stationary-document reading area on the platen glass and an image thereon is read by the image reading device while the document holding member being open.
Such external light does not enter the reading table while the document holding member is closed and covering an entire surface of the platen glass. However, in a case where an image of a document having a relatively large thickness such as book is placed on the platen glass to be read, the document holding member can not be completely closed, thereby allowing external light to enter the reading table. In another case where an image on a large-sized document is to be read, the document holding member may be purposely kept open during image reading so that an operator can see that a portion desired to be read in the document is properly positioned within the stationary-document reading area. In this case, too, external light may enter the reading table.
Where the illuminance of the light source is sufficiently high relative to external light, an amount of reflected light from the reference member is also sufficiently high, and addition of the external light to the reflected light does not lead to significant degradation in the quality of the read image. However, with decrease in the illuminance of the light source and accordingly the reflected light from the reference member, the ratio of the external light to the reflected light increases. Hence, when external light affects the adjustment of the amount of the light emitted from the light source (which may be referred to as “the light amount adjustment” hereinafter), namely, where the external light is entering the reading table during the light amount adjustment is performed, the illuminance of the light source of the image reading device, or the amount of the light emitted from the light source, is adjusted to a value that is smaller than when external light does not affect the light amount adjustment, by an amount corresponding to the external light. The amount of the light with which the document is irradiated to be read thereby accordingly decreases, making the read image blackish or darker than expected. In particular, in the case where the image reading device is moved to scan or read the document fixed in position on the stationary-document reading area, the influence of external light on the read image gradually reduces in a lateral direction from the side of the fed-document reading area to the opposite side, and thus the degradation in the quality of the read image is more significant in this case. JP-A-2003-134307 discloses a technique for resolving this problem, that is, shield plates are disposed on the laterally opposite sides of the image reading device to prevent incidence of external light on the light receiving element of the image reading device.
The shield plates can prevent incidence of external light on the image reading device in a direction from each of the lateral sides of the image reading device, but can not prevent entrance of external light into the reading table in a direction of a thickness of the platen glass. In particular, where a width of the partitioning member is made small for reducing the overall size of the image reading apparatus, or where the platen glass is constituted by a single glass plate, external light adversely affects the light amount adjustment, i.e., the adjustment of the light amount with respect to the reference member.
Hence, it is necessary to detect external light entering the reading table when the light amount adjustment for the image reading device is made and when an image is read. Although entrance of external light into the reading table is detectable by a sensor disposed for directly detecting the entering external light, or for detecting opening/closing of the document holding member, disposing such a sensor requires a space and goes against the existing demand to reduce the overall size of the image reading apparatus, as well as leads to increase in the number of components and assembly steps that in turn increases the cost of the apparatus.